1. Field of the Invention
This invention relates to a frequency diversity receiving system based on cancellation of a C/A code (Clear/Acquisition code) in the GPS (Global Positioning System).
2. Description of the Prior Art
The GPS is originally a position measuring system employing satellites which has been constructed by the United States of America for the purpose of providing fast-travelling objects such as airplanes, ships, vehicles or the like with a means of knowing their positions and velocities in real-time.
In this position measuring with the GPS, the distances between the satellites and a signal-receiving point are first measured by using radio waves of L.sub.1 band and L.sub.2 band which are transmitted from the satellites, those radio waves containing navigation information on orbits, time, correction factor for the ionospheric layer, or the like. Then, on the basis of the positions of the satellites, the position of the signal-receiving point is geometrically calculated from the distances between the satellites and the signal-receiving point.
These radio waves carry two kinds of spread spectrum signals which are modulated by a C/A code and a P code (Precision code), respectively. The spread spectrum signals of the P code are, however, intended for military uses and are not open to the use by the public users at large. Therefore, the public users are at present obliged to perform the measuring on the basis of the spread spectrum signals of the C/A code of L.sub.1 band.
However, when measuring is performed on the basis of the spread spectrum signals of the C/A code as mentioned above, the signals are delayed when they are transmitted through the ionospheric layer, resulting in measuring errors. Although correction factors are, therefore, used to correct the delays, the errors could not surely be eliminated.
On the other hand, the spread spectrum signals of the P code are intended to correct the delays in transmission through the ionospheric layer in real-time. However, as previously mentioned, the public users at large cannot use them because they are not open to the public use.
Accordingly, it is an object of this invention to provide a system in which measurements can be performed at a higher accuracy by using the GPS.
This invention has been made by paying attention to the following points, i.e.,
that spread spectrum signals of the same P code are carried in spread-modulated in the same phase in spread spectrum signals of L.sub.1 band and L.sub.2 band which are transmitted from the GPS satellites, and
that, in order to demodulate navigation information for position measuring, spread spectrum signals of the C/A code in L.sub.1 band must be received.